Kai Nielsen (May 15, 1926 – April 7, 2021) was an American professor, latterly emeritus, of philosophy at the University of Calgary. He specialized in naturalism, metaphilosophy, ethics, analytic philosophy, social and political philosophy. Nielsen also wrote about philosophy of religion, and was an advocate of contemporary atheism. He was also known for his defense of utilitarianism, writing in response to Bernard Williams's criticism of it.